How to avoid duplicate in ArrayList?

Question

I have used a for loop to add the student details to ArrayList. When I give second student details, it overwrites the first data. Stud class

import java.util.Scanner;
public class Stud {
    private int rollNumber, age;
    private String firstName, lastName, gender;
    public Stud(int rollNumber, String firstName, String lastName, int age, String gender) {

    }
    public void setRollnumber(int rollNumber) {
        this.rollNumber=rollNumber;
    }
    public int getRollnumber() {
        return rollNumber;
    }
    public void setFirstname(String firstName) {
        this.firstName=firstName;
    }
    public String getFirstname() {
        return firstName;
    }
    public void setLastname(String lastName) {
        this.lastName=lastName;
    }
    public String getLastname() {
        return lastName;
    }
    public void setAge(int age) {
        this.age=age;
    }
    public int getAge() {
        return age;
    }
    public void setGender(String gender) {
        this.gender=gender;
    }
    public String getGender() {
        return gender;
    }
}

Main class to add the data to ArrayList.

import java.util.ArrayList;
import java.util.Scanner;


public class MainClass {

    public static void main(String[] args) {
        ArrayList<Stud> al=new ArrayList<Stud>();
        System.out.println("Enter the details:");
        for(int i=1;i<=10;i++) {
        int rollNumber;
        String firstName;
        String lastName;
        int age;
        String gender;
        Scanner in=new Scanner(System.in);      
        rollNumber=in.nextInt();
        firstName=in.next();
        lastName=in.next();
        age=in.nextInt();
        gender=in.next();

        Stud s=new Stud(rollNumber, firstName, lastName, age, gender);

            al.add(s);

            for (Stud stud : al) {
                stud.setRollnumber(rollNumber);
                System.out.println(stud.getRollnumber());
            }
        }           

    }

}

Output: actual output for roll number [1] after second data [2,2]. Expected output should be Roll Number: A1 Name: F1 L1 Gender: Male Age: 11 Sports Event: Javelin

Eligibility: No (Since Javelin is open for the students 15 and above)

Roll Number: A2 Name: F2 L2 Gender: Female Age: 14 Sports Event: 100 meter run

Eligibility: Yes

Roll Number: A3 Name: F3 L3 Gender: Male Age: 11 Sports Event: 50 meter run Eligibility: No (Only for girls ageing 10 to 12)

Answer 1

Based on what you asked,Hope this works. It doesn't have problem with overwriting of previous record. Use Iterator class to add records to the arraylist.

import java.util.*;
import java.util.ArrayList;
public class ArraLis {

    private int rollNumber, age;
    private String firstName, lastName, gender,event;
    ArraLis(int rollnum, String fname, String lname, int ag, String gend,String even) {
        rollNumber=rollnum;
        age=ag;
        gender=gend;
        firstName=fname;
        lastName=lname;
        event=even;
    }
    public void setRollnumber(int rollNumber) {
        this.rollNumber=rollNumber;
    }
    public int getRollnumber() {
        return rollNumber;
    }
    public void setFirstname(String firstName) {
        this.firstName=firstName;
    }
    public String getFirstname() {
        return firstName;
    }
    public void setLastname(String lastName) {
        this.lastName=lastName;
    }
    public String getLastname() {
        return lastName;
    }
    public void setAge(int age) {
        this.age=age;
    }
    public int getAge() {
        return age;
    }
    public void setGender(String gender) {
        this.gender=gender;
    }
    public String getGender() {
        return gender;
    }
public static void main(String args[])
{
    Scanner in=new Scanner(System.in);
    ArrayList<ArraLis> al=new ArrayList<ArraLis>();
    System.out.println("enter the details");
    int rollNumber;
    String firstName;
    String lastName;
    int age;
    String gender;
    String event;
    int i;
    for(i=0;i<=2;i++)
    {
    rollNumber=in.nextInt();
    firstName=in.next();
    lastName=in.next();
    age=in.nextInt();
    gender=in.next();
    event=in.next();


    ArraLis a1=new ArraLis(rollNumber, firstName, lastName, age, gender, event);
    al.add(a1);

    }
    Iterator it=al.iterator();
    while(it.hasNext())
    {
        ArraLis ar=(ArraLis)it.next();
        System.out.println(ar.rollNumber+" "+ar.firstName+" "+ar.lastName+" "+ar.age+" "+ar.gender+" "+ar.event);
    }



}
}

Answer 2

If you are able to use the Stream class in Java 8, you can use distinct:

list.stream().distinct().collect(Collectors.toList());

An ´equals`-method for your objects is essential.

Answer 3

That should work:

public class MainClass {

   public static void main(String[] args) {
       Set<Stud> al=new HashSet<>(); // use a set
       Scanner in=new Scanner(System.in); // init scanner here!
       System.out.println("Enter the details:");
       for(int i=1;i<=10;i++) {
           int rollNumber;
           String firstName;
           String lastName;
           int age;
           String gender;

           rollNumber=in.nextInt();
           firstName=in.next();
           lastName=in.next();
           age=in.nextInt();
           gender=in.next();

           Stud s=new Stud(rollNumber, firstName, lastName, age, gender);

           al.add(s);

           for (Stud stud : al) {
               System.out.println(stud.getRollnumber());
           }
       }
   }

private static class Stud {
    private final int rollNumber, age;
    private final String firstName, lastName;
    private final String gender;

    // assign the values you get in constructor
    public Stud(int rollNumber, String firstName, String lastName, int age, String gender) {
        this.rollNumber = rollNumber;
        this.firstName = firstName;
        this.lastName = lastName;
        this.age = age;
        this.gender = gender;
    }

    public int getRollnumber() {
        return this.rollNumber;
    }
    public String getFirstname() {
        return this.firstName;
    }
    public String getLastname() {
        return this.lastName;
    }
    public int getAge() {
        return this.rollNumber;
    }
    public String getGender() {
        return this.gender;
    }

    // needed for Set
    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + this.age;
        result = prime * result + ((this.firstName == null) ? 0 : this.firstName.hashCode());
        result = prime * result + ((this.gender == null) ? 0 : this.gender.hashCode());
        result = prime * result + ((this.lastName == null) ? 0 : this.lastName.hashCode());
        result = prime * result + this.rollNumber;
        return result;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Stud other = (Stud) obj;
        if (this.age != other.age)
            return false;
        if (this.firstName == null) {
            if (other.firstName != null)
                return false;
        } else if (!this.firstName.equals(other.firstName))
            return false;
        if (this.gender == null) {
            if (other.gender != null)
                return false;
        } else if (!this.gender.equals(other.gender))
            return false;
        if (this.lastName == null) {
            if (other.lastName != null)
                return false;
        } else if (!this.lastName.equals(other.lastName))
            return false;
        if (this.rollNumber != other.rollNumber)
            return false;
        return true;
        }
    }
}